Machine tool timer



July 25, 1-967 B. H. KLYCE MACHINE TOOL TIMER 2 Sheets-$heet 1 Filed June 50, 1964 INVENTOR. BariZZe H [1 [ya Blair 4% Baa/(Z65 HTZWRNEY'S'.

July 25, 1967 B. H. KLYCE MACHINE TOOL TIMER 2 Sheets-Sheet 2 Fild June so,- 1964 s Y e E my km N Mm WK ar mfl 3H m 6 U w m a Z 3m B 0 o United States Patent 3,333,175 MACHINE TOOL TIMER Battle H. Klyce, Stamford, Conn, assignor to Regent Controls, Inc., Stamford, Conn. Filed June 30, 18964, Ser. No. 379,105 8 Claims. (Cl. 318-487) The present invention relates to a timer and, more particularly, to an electronic timer employing solid-state circuitry. The novel timer embodying the present invention is specifically adapted for short cycle timing functions having particular application in controlling high speed automated machine tool and processing operations.

Heretofore, the most widely used machine tool timer has been of the pneumatic type. Such timers employ a movable diaphragm which is initially deflected from its mid-position to begin the timing of a desired time period. The diaphragm, carrying a switch closure member, is urged toward its mid-position by spring pressure. The return of the diaphragm to its mid-position is resisted by an air pressure chamber from which air escapes through a metering valve. The time required for the diaphragm to return to its mid-position, and thereby complete a circuit to energize a motor control relay at the expiration of the time period, is varied by adjustment of the metering valve.

Pneumatic timers, however, have not been found to be particularly accurate, and they suffer from the distinct disadvantage of lacking repeatability. Repeatability refers to the ability to describe uniform time periods with a constant timer adjustment. It has been found with pneumatic timers that, for a constant setting, the actual time period described may vary quite widely.

Since pneumatic timers require a mechanical linkage between the diaphragm and the switch closure member completing the energizing circuit for the motor control relay, remote location of the timer from the switch closure member is highly impractical. In addition, adjustment of the timer from a remote location, as for example the front of a control console, is also highly impractical.

Moreover, pneumatic timers are not particularly long lived, and it is also found that the adjustment dial controlling the metering valve can not be linearly calibrated. Pneumatic timer operation is subjected to atmospheric variations. In addition, such timers are highly inaccurate in timing short time periods.

Another type of motor timer employs a synchronous motor whose output shaft is coupled through an arrangement of clutches and gear trains to a switch closure memher. The time period described by the synchronous motor timer corresponds to the time required for the switch closure member, as driven by the motor output shaft, to rotate from an initial position to switch closure position. Such timers require a considerable number of moving parts and are thus not very long lived and need constant maintenance. For describing short time periods, synchronous motor timers are found to be rather inaccurate due to gear backlash and clutch slippage. Moreover, timers of this type have a relatively long duty cycle; that is, they require a long time to recover between consecutive timing cycles. Furthermore, such timers are of relatively large physical size and thus present mounting and space problems when incorporated in a control console.

Certain electronic timers have been used in the past but have not gained wide acceptance because of their relatively high cost. Generally, these electronic timers have used vacuum tubes which necessarily increase the timers physical size, require a warm-up time and present heat dissipation problems. Moreover, prior electronic timers require the use of a fragile peanut relay to complete the energizing circuit to the motor control relay.

3,333,175 Patented July 25, 1967 "ice It is therefore an object of the present invention to provide an electronic control circuit employing solid-state circuitry and no moving parts.

It is an additional object to provide a timing control circuit of the above character for achieving greater timing accuracy and faster recycling between timing functions than heretofore possible.

An additional object is to provide a wide range electronic timer of the above character which is compact in size, low in cost and high in reliability.

A further object is to provide a timer of the above character which is readily adapted for adjustment from a remote location to describe varying time periods.

An additional object is to provide an electronic timer of the above character for directly controlling the operation of a motor control relay without employing intermediary switching means.

A still further object is to provide a timer of the above character which is particularly adapted to timing the operation of industrial machine tools.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which:

FIGURE 1 is a detailed circuit schematic diagram of one embodiment of the present invention;

FIGURE 2 is a detailed circuit schematic diagram of another embodiment of the present invention which is constructed by substituting the circuitry enclosed in dashed box 88 for the circuit portion of FIGURE l enclosed in dashed box 87; and

FIGURE 3 is a detailed circuit schematic diagram of still another embodiment of the present invention.

Similar reference characters refer to similar parts throughout the several views of the drawings.

Broadly stated, the invention comprises a solid-state electronic timer which is to be connected directly in a series energization circuit with the operating coil of a motor control relay where both the timer and the coil are connected across a power supply. A switch is operated to connect the energization circuit to the power supply and thereby initiate a time period. During the period to be timed, the timer constitutes a high impedance network as compared to the relay coil so as to maintain the energization level in the energization circuit below the level required to operate the control relay.

At the conclusion of the time period, the timer is automatically converted to a low impedance network relative to the impedance of the relay coil thus developing sufficient voltage drop across the relay coil to effect operation of the motor control relay. The relay coil, once operated, effects contact closure and circuit completion between the power supply and a load, such as the field windings of an electric motor.

In one embodiment, unique holding circuitry is incorporated to maintain an electronic switch in its low impedance state so as to sustain the low impedance condition of the timer and thus continued operation of the control relay. In the other disclosed embodiments, the control relay also operates secondary contacts to complete a holding current path shunting the timer to insure continued operation of the control relay coil and therefore the continued primary switch closure between the load and the power supply.

Turning to FIGURE 1, the electronic timer, generally indicated at 10, is connected at terminal 12 to one side of an A.C. power supply on line 13 through a switch 14. Switch 14 may be manually operated or automatically operated to closure in synchronization with related controlled functions to initiate the time period to be metered by the electronic timer 10. A second output terminal 16 is connected to the other side of the AC. power on line 17 through a control relay coil 18. It is thus observed that the electronic timer 10 is connected directly to and in series with the relay coil 18.

A relay plunger, indicated schematically at 20, carries normally-open primary relay contacts 22 and normallyopen secondary contacts 24. On operation of the control relay at the expiration of the time period determined by the timer 10, the plunger 20 is attracted by the relay coil 18 to close relay contacts 22 to energize a motor 26 from the AC. source and to close contacts 24 for completing a holding current path shunting the electronic timer for continued energization of the relay coil. The motor 26 may be adapted to power a machine tool or control a processing operation. To de-energize the motor 26, the switch 14 is opened manually or automatically in accordance with related controlled functions.

Considering the electronic timer 10 in detail, a fullwave rectifying bridge comprising diodes 28, 30, 32 and 34 is connected between terminals 12 and 16. The line 36 between the cathodes of diodes 28 and 32 and the 38 line between the anodes of diodes 3t) and 34 constitute the output terminals for the full-wave rectifying bridge. Accordingly, the network connected between lines 36 and 38 is energized by a unidirectional pulsating current, with the line 36 retaining a positive polarity and the line 38 retaining a negative polarity. It will thus be seen that the network connected between lines 36 and 38 is electrically in series with the relay coil 18.

A pair of resistors 40 and 42 along with a potentiometer 44, which may be remotely located, are connected in series with a timing capacitor 46 to provide a high impedance circuit path between lines 36 and 38. A Zener diode 48 is connected across the series combination of potentiometer 44 and capacitor 46 between junction 49 and line 38 in order to maintain a constant maximum voltage drop across these two circuit elements regardless of fluctuating line voltage.

A filtering network comprising the parallel combinations of a capacitor 50 and a resistor 52, and a capacitor 54 and resistor 56 is connected between line 38 and the junction 58 between resistors 40 and 42; the latter having a large resistance value compared to the former. This filtering network serves to smooth out the rectified DC. voltage appearing at junction 58.

In addition, an organ tube 60 is connected in series with a large resistor 62 between the line 38 and the junction 58 between resistors 40 and 42.

The series combination of a diode 64 and silicon controlled rectifier 66 comprises a low impedance circuit path between lines 36 and 38 when the controlled rectifier 66 is conducting. The junction 68 between the potentiometer 44 and the capacitor 46 is connected through a neon tube 70 to the gate input 72 of the controlled rectifier 66. The junction 74 between the neon tube 70 and the gate input 72 of the controlled rectifier 66 is connected to the line 38 through a resistor 76. In addition, the junction 68 is connected through a diode 78 and resistor 80 to the junction 82 between the cathode of diode 64 and the anode of controlled rectifier 66. The junction 82 is then returned to line 38 through a large resistor 84.

In operation, when the switch 14 is initially closed to initiate a time period, the controlled rectifier 66 is in its blocking characteristic state to effectively open the low impedance circuit path between lines 36 and 38. Thus, the impedance of the timer 10, is determined by the parameters of the high impedance circuit path. between lines 36 and 38 consisting of resistors 40 and 42, potentiometer 44 and capacitor 46. The impedance of this path, being in series with the relay coil 18, is designed to be sufiiciently large so as to limit the voltage drop'across the relay coil to a value less than the pull-in voltage necessary to operate the control relay. In addition, the impedances of this path should limit the inrush current to prevent operation of the control relay immediately on closure of switch 114.

The capacitor 46 begins charging toward the voltage appearing at junction 49 which is maintained at a constant magnitude by Zener diode 98. Once the voltage across capacitor 46 appearing at junction 68 reaches the threshold voltage necessary to cause the neon tube 70 to conduct, the resulting potential developed across the resistor 76, as applied to the gate input 72, causes the silicon controlled rectifier 66 to switch to its conducting state. Thus, the high impedance circuit path is shunted by the low impedance path including the diode 64 and the conducting controlled rectifier 66. As a result, the impedance of the timer 10 drops to an extremely low value relative to the impedance of the relay coil 18, increasing the voltage drop across the relay coil to a magnitude exceeding the pull-in voltage necessary to operate the control relay.

Once the voltage across the capacitor. 46 exceeds the threshold voltage to cause conduction of neon tube 70, the capacitor begins discharging through the neon tube. As the voltage at the junction 68 falls below approximately 50 volts, the turn-off voltage for neon tube 70, this discharge path is broken. A secondary discharge path for the capacitor 46 is provided, however, through diode 78, resistor and the low impedance anode-cathode circuit of controlled rectifier 66. This discharge path serves to provide sufiicient holding current flow through the silicon controlled rectifier 66 to maintain its conducting state for a sufiicient period of time to insure operation of the control relay.

Since the silicon controlled rectifier 66, once triggered to its conducting state by a gating input at 72, remains conducting only so long as its anode remains at a positive potential relative to its cathode, the rectifier will regain its blocking characteristics on the occurrence of a zero voltage node in the rectified DC. voltage appearing on line 36; the gating input having terminated when the voltage at junction 68 fall below the turn-off voltage of the neon tube 70. In such event the timer 10 may revert to its high impedance condition before the control relay can establish its holding circuit through closure of the normally open secondary relay contacts 24. However, by the time the discharge current through the controlled rectifier 66 falls below the value of the holding current necessary to maintain the rectifier in its conducting state, the control relay will have had suflicient time to operate so as to close its holding circuit contacts 24 shunting the timer 10. Any charge remaining on the capacitor 46 after the controlled rectifier 66 has regained its blocking characteristics will be ultimately dissipated through the resistor 84.

The provision of the diode 64 serves to prevent the capacitor 46 from distributing its charge to the capacitors 50 and 54 in the filtering network.

An additional important feature of the invention resides in the use of the argon tube 60 which is continuously energized as long as the timer 10 is connected to the power supply through switch 14. In wiring the circuit for the electronic timer 10, the neon tube 7 i), being photosensitive, is mounted in optical coupling relationship with the argon tube 60, which is photoernissive, in order to establish a constant ambient light condition and thereby stabilize the neon tube at a constant turn-on voltage corresponding to the required threshold voltage for operation of the controlled rectifier 66.

It is thus seen that the charging rate of the capacitor 46 determines the duration of the time period initiated by closure of switch 14 and terminated when the voltage across the capacitor exceeds the threshold voltage of neon.

tube 70. This charging .rate is readily varied by appropriate adjustment of the potentiometer 44. The range of adjustment is determined by appropriate selections of impedance values for the potentiometer 44 and timing capacitor 46. Moreover the parameters of the discharge path for capacitor 46, particularly the resistance value of resistor 80, are selected so as to provide the requisite duration of holding current for controlled rectifier 66 and yet permit fast recycling, if desired. It will be appreciated that the capacitor 46 must be completely discharged when the switch 14 is closed for the timer to accurately time a particular time period.

Turning to the modification shown in FIGURE 2, a unijunction transistor 86 is utilized to establish the threshold voltage which must be exceeded to fire the controlled rectifier 66. Accordingly, the unijunction transistor 86 performs the function of the neon tube 70 of FIGURE 1. To replace the neon tube 70 with the unijunction transistor 86 in the timer 10, the portion of the circuit of FIG- URE 1 enclosed in the box 87 is replaced by the portion of FIGURE 2 enclosed in box 88. Identical circuit parts of FIGURES 1 and 2 are given corresponding reference numerals. I

As seen in FIGURE 2, the emitter 86a of unijunction transistor 86 is connected through a resistor 90 to the junction 68 between timing capacitor 46 and potentiometer 44. The base one terminal 86b of unijunction transistor 86 is connected through a resistor 92 to the line 38 while its base two terminal 86c is connected through a resistor 94 to a junction 95 common to junction 49 at the upper terminal of the potentiometer 44. A capacitor 96 is connected across the unijunction transistor 86 from junction 95 to line 38. A diode 98 and a resistor 100 are series connected between junction 95 and junction 82 at the anode of the controlled rectifier 66. A capacitor 102 is connected between line 38 and the junction 183 of the diode 98 and resistor 100. The junction 104 between the base one terminal 86b of unijunction transistor 86 and resistor 92 is connected to the gate input terminal 72 of controlled rectifier 66.

The operation of the timer 10 incorporating the modification of FIGURE 2 is basically identical to that previously described. Initially, the emitter-base one circuit of the unijunction transistor 86 is virtually an open circuit as long as the voltage difference between the emitter terminal 86a and base one terminal 86b is less than a critical value. This is directly analogous to the neon tube 70 of FIG- URE l which does not conduct until the voltage drop across it exceeds a threshold or critical value. Typically, the voltage at the emitter terminal 86a must exceed 60% of the voltage across the base terminals of the unijunction 86 before the emitter-base one circuit breaks down and conducts.

It will thus be seen that the voltage applied to the emitter 86a of unijunction transistor 86 is that which is developed across the timing capacitor 46. Accordingly once the timing capacitor 46, in charging toward the maximum voltage established by the Zener diode 48 (FIG- URE 1), raises junction 68 to the threshold voltage, the unijunction transistor 86 breaks down. The timing capacitor 46 then discharges completely through the emitterbase one circuit and resistor 92. The positive voltage developed across resistor 92 fires the controlled rectifier 66 completing the low impedance path between lines 36 and 38 to signal the end of the period being timed.

To prevent the controlled rectifier 66 from regaining its blocking characteristics on the occurrence of a zero voltage node in the rectified DC. voltage appearing on line 36 before the control relay has established its holding circuit, the capacitor 102, which was charged through diode 98 to the voltage across the Zener diode 48, discharges through resistor 100 and the anode-cathode circuit of the controlled rectifier 66. This discharging current provides the requisite holding current to hold the controlled rectifier in conduction sufficiently long to insure completed operation of the control relay. Diode 98 prevents 6 the capacitor 102 from discharging through the unijunction circuits as diode 64 (FIGURE 1) prevents the dist-ribution of charge to the filtering capacitors 50 and 54 (FIGURE 1).

Capacitor 96 serves as a filter to maintain the voltage across the base terminals of the unijunction transistor 86 substantially constant notwithstanding fluctuations in the line voltage and thus prevent premature breakdown of the unijunction transistor. Since the timing capacitor 46 is effectively completely discharged through the emitterbase one circuit of the unijunction transistor 86, the bleeder resistor 84 (FIGURE 1) is not needed in this embodiment. In situations where the potentiometer 44 is adjusted to provide for small series resistances, the unijunction transistor 86 may break down prematurely because the voltage at junction 68 exceeds 60% of the voltage at junction 49 immediately on closure of the switch 14 (FIGURE 1). It is thus advisable to insert a resistor between the junction 49 and the potentiometer 44 to initially hold junction 68 below threshold voltage.

Referring now to the embodiment disclosed in FIG- URE 3, the timer 10 is connected to the motor control relay coil 18 at terminal 16 and switch 14 at terminal 12 with the resulting series combination connected across power lines 13 and 17, all in the manner of FIGURE 1. Closure of switch 14 produces a pulsating or interrupted D.C. voltage across lines 36 and 38 by virtue of the fullwave rectifying circuit arrangement of diodes 28, 30, 32 and 34.

The high impedance circuit path consisting of resistors 40 and 42, potentiometer 44, and timing capacitor 46 are connected across lines 36 and 38. The filtering network of capacitors 50 and 54, and resistors 52 and 56 are connected from junction 58 to line 38. The voltage limiting Zener diode 48 is connected from junction 49 to line 38. The function of the parts thus far described is identical to the correspondingly referenced parts of FIGURES l and 2.

Unijunction transistor 86 has its base two terminal 860 connected through resistor 94 to junction 49 and its base one terminal 86b connected through resistor 92 to line 38. Junction 68 between potentiometer 44 and timing capacitor 46 is connected through resistor to the emitter 86a of unijunction transistor 86. Capacitor 96 is connected across the unijunction transistor 86. The above described parts correspond in identity and function to the like referenced parts of FIGURE 2. The resistor connected between the potentiometer 44 and junction 49 serves to initially hold junction 68 below threshold voltage as was discussed in connection with FIGURE 2.

Unlike the embodiments of FIGURES 1 and 2, the anode of controlled rectifier 66 is connected directly to line 36 while its cathode is connected to line 38 through a small resistance 112. Contrasted with FIGURE 2, the junction 104 at the base one terminal 86b of unijunction transistor 86 is connected to the gate input terminal 72 of controlled rectifier 66 through a diode 114. Junction 116 at the cathode of diode 114 is connected through a series resistor 118 and capacitor 120 to line 38.

The operation of the timer 10 of FIGURE 3 to initially fire the controlled rectifier 66 is essentially identical to that described for the embodiment of FIGURE 2. Specifically the timing capacitor 44 charges to the threshold voltage causing the unijunction transistor 86 to break down. Timing capacitor 44 then discharges through the resistor 90, the emitter-base one circuit of unijunction transistor and resistor92, developing a positive voltage pulse across resistor 92. This positive voltage pulse is conducted through diode 114 to the gate input terminal 72, firing controlled rectifier 66.

A characteristic feature of the controlled rectifier 66 is the presence of a small voltage differential between its cathode and the gate input terminal 72 when current flows in its anode-cathode circuit. As this current approaches zero, this voltage differential also approaches zero and, under ordinary circumstances, the controlled rectifier 66 would regain its blocking characteristic with the occurrence of the next node of the interrupted DC. voltage across its anode and cathode terminals. This situation prompted the provisions disclosed in FIGURES 1 and 2 for supplying holding current to the controlled rectifier 66 in order to maintain it in conductance until the control relay could establish its own holding circuit through relay contacts 24.

In the embodiment of FIGURE 3 however, it is found that by connecting capacitor 120 between the gate input terminal 72 and the cathode of controlled rectifier 66, it is possible to store enough energy to hold the voltage at the gate input 72 sufliciently positive and thereby maintain the controlled rectifier in conductance through successive voltage nodes. During each period of current flow through the controlled rectifier 66, the charge on capacitor 120 is replenished so as to prevent the rectifier from regaining its blocking characteristics on occurrence of each succeeding voltage node on lines 36 and 38.

In practice, it is found that the capacitor 120 must be in the range of 50 microfarads for typical controlled rectifiers in order to obtain the desired operation. The resistor 118 reduces the shunting effect of the capacitor 120 on the voltage pulse developed across the resistor 92. Al-

though not absolutely necessary, it is desirable to include resistor 112 in order to enhance the normal gate-cathode voltage drop of the controlled rectifier 66 and thus in crease the charge developed on capacitor 120. Diode 114 prevents capacitor 120 from discharging through resistor 92.

It will thus be seen that the controlled rectifier 66 can be held in conductance as long as the switch 14 is closed, thus affording the distinct advantage of eliminating the necessity for a separate self-holding circuit for the control relay coil 18 as required in the embodiments of FIGURES 1 and 2. Moreover, since a supply of holding current for the controlled rectifier 66 is no longer necessary, the numerous circuit elements of FIGURES 1 and 2 for providing this holding current are not required in the embodiment of FIGURE 3. It will also be seen that the diode 64 of FIGURE 1 can thus be omitted in the embodiment of FIGURE 3.

The use of the embodiments shown in FIGURES 2 and 3 are found to give the timer greater stability and better repeatability than the embodiment of FIGURE 1 which uses the neon tube 70. Moreover, the use of the unijunction transistor 86 provides for a smaller and less expensive timer which is insensitive to light.

The invention thus provides an electronic machine tool timer capable of highly reliable and extremely accurate operation. Long life is achieved through the use of solid state circuitry. The timer can be directly connected in the energizing circuit for any control relay, contactor or motor starter through Nema Size 1. The timer can be encapsulated in a package occupying as little as 6 cubic inches of space.

The potentiometer 44 can be located remotely from the timer itself so as to permit adjustment at the front panel of a control console, for example. The timer can be adapted to meter any fixed time period from 0.05 second to in excess of 100 seconds with a high degree of repeatability.

Although the timer has been disclosed in combination with an A.C. power supply, it will be appreciated that a DC. power source could be used and, in such a situation, the full-wave bridge and the provisions for holding the controlled rectifier 66 in conduction would be unnecessary.

Although the timer has particular application in the machine tool and process control fields, it may be used in any number of applications where time controlled functions are desired. Moreover, the timer 10 is not limited to motor control applications; as it may be used to achieve time controlled operation of other electromechanical devices, such as solenoid operated valves.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention which, as a matter of language, might be said to fall therebetween.

Having described by invention, what I claim as new and desire to secure by Letters Patent is:

1. A time control system comprising (A) an A.C. energizing source,

(B) an AC. motor,

(C) a switch operating to initiate a time period,

(D) a control relay including (1) an operating coil,

(2) normally open primary relay contacts establishing an energizing circuit between said source and said motor on operation of said relay, and

(3) normally open secondary relay contacts establishing a holding circuit 'for said coil from said source on operation of said relay and (B) an electronic timer connected in series with said said coil and said switch and functioning to effect operation of said relay at the expiration of a pre determined time period, said timer including (1) a full-wave rectifying bridge connected through said switch to said source,

(2) a high impedance circuit path connected between output terminals of said bridge, said high impedance path including (a) a timing capacitor (i) charging toward the voltage of said bridge on operation of said switch to initiate a time period;

'(ii) at a rate determined by the parameters of said high impedance circuit,

(3) a low impedance circuit path connected between said output terminals of said bridge, said low impedance path including (a) a controlled rectifier having (i) a gating terminal (4) a neon tube connected between the high voltage side of said capacitor an the gating terminal of said controlled rectifier, said neon tube (a) operating to trigger said controlled rectifier to its conducting state when the voltage across said capacitor exceeds a threshold voltage established by said neon tube, and

(5) a discharge path connected between said capacitor and the anode of said controlled rectifier to supply holding current for said controlled rectifier,

(F) whereby said timer is converted from a high impedance network to a low impedance network on expiration of said time period to increase the energization level in said coil to a level sufiicient to operate said control relay.

2. The system defined in claim 1 wherein said timer includes (6) a photoemissive source disposed in optically coupled relationship with said neon tube for establishing a constant ambient light condition to stabilize the threshold voltage characteristic of said neon tube.

3. A time control system comprising (A) an A.C. energizing source,

(B) an A.C. motor,

(C) a switch operating to initiate a time period,

(D) acontrol relay including use" (1) an operating coil,

(2) normally open primary relay contacts establishing an energizing circuit between said source and said motor on operation of said relay, and

(3) normally open secondary relay contacts establishing a holding circuit for said coil from said source on operation of said relay and (B) an electronic timer connected in series with said coil and said switch and functioning to effect operation of said relay at the expiration of a predetermined time period, said timer including (1) a full-wave rectifying bridge connected through said switch to said source,

(2) a high impedance circuit path connected between output terminals of said bridge, said high impedance path including (a) a timing capacitor (i) charging toward the voltage of said bridge on operation of said switch to initiate a time period; (ii) at a rate determined by the parameters of said high impedance circuit,

(3) a low impedance circuit path connected between said output terminals of said bridge, said low impedance path including (a) a controlled rectifier having (i) a gating terminal (4) a unijunction transistor operating to trigger said controlled rectifier to its conducting state when the voltage across said timing capacitor exceeds a predetermined threshold voltage, said unijunction transistor having (a) an emitter connected to a terminal of said timing capacitor and (b) base terminals connected across a portion of said high impedance circuit path, (i) one base terminal being connected to said gating terminal of said controlled rectifier,

(5) a capacitor connected to the anode of said controlled rectifier to supply holding current for said triggered controlled rectifier,

(F) whereby said timer is converted from a high impedance network to a low impedance network on expiration of said time period to increase the energization level in said coil to a level sufficient to operate said control relay.

4. A control system comprising, in combination:

(A) an energizing source supplying an interrupted DC.

voltage across first and second terminals,

(:B) an operating device,

(C) a silicon controlled rectifier connected in a series energization circuit with said source and said operating device, said controlled rectifier having (1) an anode connected to said first terminal,

('2) a cathode connected to said second terminal,

and

(3) a gate (D) means for selectively developing an input signal to said gate so as to switch said controlled rectifier into conduction and thereby complete said energization circuit for said operating device, and

(E) a capacitor connected between the cathode and gate of said controlled rectifier, said capacitor (1) functioning to hold said controlled rectifier in conduction during interruptions of said DC. voltage after termination of said input signal.

5. The system claimed in claim 4 wherein (E) ('2) the capacitance of said capacitor is in the order of 50 microfarads,

(F) a first resistor is connected between the cathode of said controlled rectifier and said second terminal, and

(G) a second resistor is connected in series with said capacitor.

6. An electronic timer for operating a device on the expiration of a predetermined time period, said timer being connected in a series energization circuit with the device and a switch operating to connect the energization circuit to an AC. power source and thereby initiate said predetermined time period, said timer including (A) a rectifying network having first and second output terminals; 7

(B) a high impedance circuit path connected between said first and second output terminals, said high impedance path including (1) a timing capacitor (a) a charging toward the rectified voltage across said first and second terminals (b) at a rate determined by the parameters of said high impedance circuit path;

(C) a controllable impedance circuit path connected between said first and second output terminals, said controllable impedance path including (1) a controlled rectifier having (a) an anode connected to said first output terminal, (b) a cathode connected to said second output terminal, and (c) a gate; (D) a control circuit connected to said gate and to said timing capacitor, said control circuit including (1) means establishing a constant threshold vol-tage and providing an input to said gate for operating said controlled rectifier to its conducting state when, upon the expiration of the predetermined time period, the voltage across said timing capacitor exceeds said threshold voltage, and (2) holding circuit means connected to said controlled rectifier for maintaining it in its conducting state despite nodes in the rectified voltage across said first and second terminals.

7. The timer define-d in claim 6 wherein said holding circuit means comprises (A) a capacitor connected to the anode-cathode circuit of said controlled rectifier and supplying holding current therethrough.

8. The timer defined in claim 6 wherein said holding circuit means comprises (A) a capacitor connected between the cathode and gate of said controlled rectifier.

References Cited UNITED STATES PATENTS 3,097,314- 7/ 1963 Harriman.

3,113,293 12/1963 Breese et a1. 301-885 X 3,206,650 9/1965 Miller et a1. 307-44 1 X 3,238,418 3/1966 Heft.

3,244,965 4/ 1966 Gutzwiller.

3,250,465 5/1966 Look 30 7141 X 3,252,078 5/1966 Conner 3078'8.5 X

ORIS L. RADER, Primary Examiner.

T. B. JOI-KE, Assistant Examiner. 

1. A TIME CONTROL SYSTEM COMPRISING (A) AN A.C. ENERGIZING SOURCE, (B) AN A.C. MOTOR, (C) A SWITCH OPERATING TO INITIATE A TIME PERIOD, (D) A CONTROL REALY INCLUDING (1) AN OPERATING COIL, (2) NORMALLY OPEN PRIMARY RELAY CONTACTS ESTABLISHING AN ENERGIZING CIRCUIT BETWEEN SAID SOURCE AND SAID MOTOR ON OPERATION OF SAID RELAY, AND (3) NORMALLY OPEN SECONDARY RELAY CONTACTS ESTABLISHING A HOLDING CIRCUIT FOR SAID COIL FROM SAID SOURCE ON OPERATION OF SAID RELAY AND (E) AN ELECTRONIC TIMER CONNECTED IN SERIES WITH SAID SAID COIL AND SAID SWITCH AND FUNCTIONING TO EFFECT OPERATION OF SAID RELAY AT THE EXPIRATION OF A PREDETERMINED TIME PERIOD, SAID TIMER INCLUDING (1) A FULL-WAVE RECTIFYING BRIDGE CONNECTED THROUGH SAID SWITCH TO SAID SOURCE, (2) A HIGH IMPEDANCE CIRCUIT PATH CONNECTED BETWEEN OUTPUT TERMINALS OF SAID BRIDGE, SAID HIGH IMPEDANCE PATH INCLUDING (A) A TIMING CAPACITOR (I) CHARGING TOWARD THE VOLTAGES OF SAID BRIDGE ON OPERATION OF SAID SWITCH TO INITIATE A TIME PERIOD; (II) AT A RATE DETERMINED BY THE PARAMETERS OF SAID HIGH IMPEDANCE CIRCUIT, (3) A LOW IMPEDANCE CIRCUIT PATH CONNECTED BETWEEN SAID OUTPUT TERMINALS OF SAID BRIDGE, SAID LOW IMPEDANCE PATH INCLUDING (A) A CONTROLLED RECTIFIER HAVING (I) A GATING TERMINAL (4) A NEON TUBE CONNECTED BETWEEN THE HIGH VOLTAGE SIDE OF SAID CAPACITOR AN THE GATING TERMINAL OF SAID CONTROLLED RECTIFIER, SAID NEON TUBE (A) OPERATING TO TRIGGER SAID CONTROLLED RECTIFIER TO ITS CONDUCTING STATE WHEN THE VOLTAGE ACROSS SAID CAPACITOR EXCEEDS A THRESHOLD VOLTAGE ESTABLISHED BY SAID NEON TUBE, AND (5) A DISCHARGE PATH CONNECTED BETWEEN SAID CAPACITOR AND THE ANODE OF SAID CONTROLLED RECTIFIER TO SUPPLY HOLDING CURRENT FOR SAID CONTROLLED RECTIFIER, (F) WHEREBY SAID TIMER IS CONVERTED FROM A HIGH IMPEDANCE NETWORK TO A LOW IMPEDANCE NETWORK ON EXPIRATION OF SAID TIME PERIOD TO INCREASE THE ENERGIZATION LEVEL IN SAID COIL TO A LEVEL SUFFICIENT TO OPERATE SAID CONTROL RELAY. 